The present invention relates to a coated cemented carbide cutting tool(indexable inserts) for the milling, particularly at high cutting speeds, of stainless steels of different composition and microstructure such as austenitic, ferritic, duplex, superaustenitic and precipitation hardened stainless steels but also for the milling of non-stainless steels such as low carbon steels and low and medium alloyed steels.
It is well known that for cemented carbide cutting tools used in the machining of steels, the cutting edge is worn by different wear mechanisms such as chemical and abrasive wear, but the tool edge may also fracture under a heavy intermittent cutting load resulting in so called edge chipping which is usually initiated by cracks formed perpendicularly to the cutting edge. This type of cracks are named comb cracks. Furthermore, different cutting conditions such as cutting speed, depth of cut, cutting feed rate and also external conditions, such as dry or wet machining, heavy vibrations of the work piece, etc., require a plurality of different properties of the cutting edge. For example, when applying a carbide cutting insert in the milling of a workpiece of a non-stainless steel or a stainless steel where the surface of the workpiece is covered by so called cast skin, or when milling under difficult external conditions such as heavy vibrations of the workpiece, a coated cemented carbide insert must be used where the insert includes a substrate of a tough cemented carbide grade and on the surface of the substrate, a hard and wear resistant refractory coating is deposited. The coating should be adherently bonded to the substrate and covering all functional parts of the insert. In addition, when milling a stainless steel, still another wear mechanism is active called adhesive wear which is caused by the adhesive force between the stainless steel chip and the cutting edge material. When the adhesive force grows large enough, edge chipping in the vicinity of the above mentioned comb cracks on the cutting edge will occur and, hence, the tool life will be shortened.
When using a cemented carbide cutting tool for the milling of a stainless steel at high cutting speeds (150-300 meter/min depending on the composition of the stainless steel), the thermal energy developed in the cutting edge is considerable and the entire tool edge may plastically deform. This type of wear mechanism is known as plastic deformation wear and, therefore, yet another requirement of the coated cemented carbide insert when being used at high cutting speeds, is that the selection of the carbide composition and the coating material results in a cutting edge exhibiting a high resistance to plastic deformation.
Commercial cemented carbide tools suitable for the machining of stainless steels and, in particular, carbide tools suitable for the milling of stainless steels are usually only optimized with respect to one or two of the required tool properties mentioned above i.e. high resistance to chemical, abrasive, adhesive and plastic deformation wear of a tough cemented carbide substrate coated with a wear resistant and an adherently bonded coating.
WO 97/20083 discloses a coated cemented carbide cutting tool particularly designed for the wet and dry milling of workpieces of low and medium alloyed steels or stainless steels, with or without abrasive surface zones, in machining operations requiring a high degree of toughness of the carbide cutting edge. The external cutting conditions are characterized by complex shapes of the workpiece, vibrations, chip hammering, recutting of the chips etc. The described cutting insert comprises a coated cemented carbide substrate containing WC with an average grain size of 1.7 .mu.m together with cubic carbides and 11-12 wt % Co, a coating including a layer of TiC.sub.x N.sub.y O.sub.z with a columnar grain structure, a second layer of a smooth, finegrained .kappa.-Al.sub.2 O.sub.3 and an outermost third layer of TiN.
WO 97/20081 discloses a coated cemented carbide cutting tool particularly designed for the wet and dry milling of low and medium alloyed steels. The described cutting insert comprises a coated cemented carbide substrate containing WC, cubic carbides and Co and a coating including a layer of TiC.sub.x N.sub.y O.sub.z with a columnar grain structure, a second layer of a smooth, finegrained .kappa.-Al.sub.2 O.sub.3 and an outermost third layer of TiN.
WO 97/20082 discloses a coated cemented carbide cutting tool particularly designed for the wet turning of stainless steel components in machining operations requiring a high degree of toughness of the carbide cutting edge. The described cutting insert comprises a coated cemented carbide substrate with a cobalt binder phase enriched in W, a coating including a layer of TiC.sub.x N.sub.y O.sub.z with a columnar grain structure, a second layer of .kappa.-Al.sub.2 O.sub.3, and an outermost third layer of TiN. A very smooth cutting edge surface is optionally obtained by brushing the tool edges with brushes based on e.g. SiC.